1. Field of the Invention
The present invention relates to a pneumatic radial tire, more particularly an improvement in the internal structure in the tread portion which is capable of reducing noise during running without sacrificing running performance.
2. Description of Related Art
Recently, there is a great demand for a low noise tire in various categories, such as passenger tires, RV tires, 4 WD car tires and the like.
In order to reduce the running noise of a tire, many efforts have been made to improve the tread pattern especially in case of the passenger tires. However, in the case of RV tires, 4 WD car tires and the like, the tread pattern of which is defined by wide grooves and circumferentially separated tread elements such as blocks, it is difficult to reduce the running noise by improving only the tread pattern. Therefore, an attempt has been made to decrease the hardness of the tread rubber, decrease the topping rubber for the belt, and/or decrease the volume of the tread grooves.
However, when the tread rubber hardness is decreased, the cornering power decreases. As a result, the steering stability is deteriorated.
When the belt topping rubber hardness is decreased, similar problems arise because the reinforcing effect of the belt decreases.
If the tread groove volume is decreased, the wet grip performance and braking performance are deteriorated.
Such deterioration in the running performance is especially remarkable in a snow tire and mud & snow tire. Therefore, it is not preferable to employ the above-mentioned methods although the running noise of such a tire is larger than that of the passenger tires. Thus, the running noise can not be fully reduced.
The above-mentioned noise reducing methods are generally for decreasing the amplitude of the generated noise. However, even if the generated noise is reduced, or the amplitude of vibrations of the tread portion is reduced, it is liable to be amplified by resonance of the tire when transmitted from the tread portion to the sidewall portion. In the frequency range of about 1000 to 1250 Hz, such a phenomenon is remarkable, and as a result, we feel that the running noise is not reduced since the human ears are sensitive to this frequency range.
Further, if a rubber layer is disposed between the plies of a tread reinforcing belt, the running noise maybe decreases. However, as the belt stiffness decreases, the cornering power is greatly decreased, and running performance such as the steering stability is greatly deteriorated.